Optical fibers find many uses for directing beams of light between two points. Optical fibers have been developed to have low loss, low dispersion, polarization maintaining properties and can be incorporated into several different types of devices, such as amplifiers, filters, lasers and interferometers. As a result, optical fiber systems find widespread use, for example in optical communications.
However, one of the important advantages of fiber optic beam transport, that of enclosing the optical beam to guide it between terminal points, is also a limitation. There are several optical components, important for use in fiber systems or in fiber system development, that are not implemented in a fiber—based form where the optical beam is guided in a waveguide. Instead, these optical components are implemented in a bulk form, and through which the light propagates freely. Examples of such components include, but are not limited to, filters, isolators, circulators, polarizers, switches and shutters. Consequently, the inclusion of a bulk component in an optical fiber system necessitates that the optical fiber system have a section where the beam path propagates freely in space, rather than being guided within a fiber.
Free space propagation typically requires use of collimation units at the ends of the fibers to produce and receive collimated beams. In some units, the same focusing element is used to collimate the beams from two different fibers placed at different positions relative to the axis of the focusing optic. This produces collimated beams that propagate in non-parallel directions. The non-parallel propagation of the collimated beams introduces extra issues for aligning the components of the device, and may place some limits on making the device smaller in size.
A fiber optic tap is a device that splits off a fraction of the light propagating in the optical fiber, typically for monitoring the power in the optical signal. In conventional approaches, the light is split from a first fiber to a second fiber spliced together with the first fiber, for example a fused biconic taper. The split ratio may be tailored so that a prescribed fraction of the incident signal is directed along the second fiber, with the remaining majority of the signal passing along the first fiber. The second fiber is typically directed to a photodetector. One problem with this approach is that the fraction of the light coupled into the second fiber may not be constant over all wavelengths of interest, and may suffer from undesirable polarization dependent effects. Another problem with this approach is that the fiber-based device requires a certain minimum fiber bend radius, which places constraints on the minimum size of the package.